The present invention relates to safety interlock devices responsive to fluid pressure in a system, and, more particularly, to safety interlock devices for inadvertently preventing the opening of an industrial pressure vessel containing a hazardous gas.
Various types of vessels are utilized for storing or transmitting hazardous gas. In a typical industrial installation, a cylindrical-shaped vessel may be provided with a wall-sized end cap. Such vessels may contain hydrogen sulfide, phosgene, or other hazardous fluids. Special safety precautions are obviously recommended to prevent the inadvertent release of gas, since the consequences of a leak may be life-threatening.
In order to accomplish inspection, maintenance, or repair operations, it may be necessary to remove the hazardous gas from the vessel and unthread the end cap for vessel access. In such a situation, cap removal is accomplished with an enormous cap retainer and threading device, and the operation, once commenced, cannot be easily and quickly reversed. If even low-pressure hazardous gas is present in the vessel, the release of gas during the cap unscrewing operation would subject nearby personnel to high risk. A conventional pressure indicator may be utilized to determine gas pressure or system pressure in such a vessel. Because of the hazardous nature of the gas, operating personnel are instructed not to unscrew the cap unless the pressure indicator reads "0" pressure.
In theory, such a pressure indicator should be sufficient to insure that no pressure is within the vessel, so that the cap removal operation may proceed without risking a hazardous gas exposure to nearby personnel. In practice, however, such a pressure indicator may provide insufficient assurances for operator safety in view of the hazardous nature of the gas. In particular, the pressure indicator may be defective and the reading inaccurate, or the indicator might not be sufficiently responsive to produce an accurate reading at a very low pressure, e.g., 2 psi. Moreover, even if the pressure indication reading were accurate, cap removal operators may have reason to doubt the reliability of the reading, or may erroneously assume that there is no pressure in the vessel and therefore neglect to read the indicator before commencing the cap removal operation. The interlocking device of the present invention is designed to provide additional protection against cap removal when the vessel contains either a high or relatively low pressure fluid.
The concept of a pressure-responsive interlock device is known in the art, as illustrated in the brochure entitled "MODCO Pipeline Hinged Closures", No. 1-84 5M, distributed by Modco Industries, Inc. The above interlock has, however, disadvantages which limit its acceptance in the industry. When under high system pressure, it may be very difficult, although possible, to manually override the interlock and open the cap. More importantly, the effort required to deactivate the interlock device when under low system pressure, e.g., 1-5 psi, is non-existent or barely existent Should the operator observe a minimal resistance upon moving the cover down, he must subjectively determine whether that resistance is due to a low fluid pressure in the vessel, or due to increased friction of moving parts because of corrosion or "plugging" of the device. Also, the same motion is utilized to both deactivate the interlock and hopefully notice the resistance presumably indicative of low pressure gas, and this downward cover movement may occur so rapidly that no operator thought occurs while the action is accomplished. The device provides no visual indication that system pressure has moved the device into the interlock position, so that the deactivation procedure is attempted regardless of the system pressure in the vessel. The device as described above can thus be intentionaly or unintentionally manually overriden, thus obviating its interlock capability and subjecting cap removal operators to high risk.
The disadvantages of the prior art are overcome by the present invention, and an improved pressure responsive safety interlock is hereinafter disclosed for preventing the opening of a closure. The device has particular utility for industrial pressure vessels containing hazardous gases, and provides additional safety for operators so that the vessel cap will not be inadvertently removed when the vessel contains a pressurized gas.